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Aerostat
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Categories of Aircraft
Supported by Lighter-Than-Air Gases (aerostats)
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An aerostat is a lighter than air object that can stay stationary in the air. Aerostats include free balloons, airships, moored balloons and tethered Helikites. An aerostat's main structural component is its envelope, a lightweight skin containing a lifting gas[1][2] to provide buoyancy, to which other components are attached.

Technically, aerostats are capable of providing "aerostatic" lift in that the force upwards arises without movement through the surrounding air mass. This contrasts with aerodynamic lift which requires the movement of at least some part of the aircraft through the surrounding air mass. However, in reality most aerostats (except spherical balloons) obtain lift from both aerodynamic lift and pure gas lift at some time or other.

The word aerostat was originally French and is derived from the Greek aer (air) + statos (standing).

Contents

Types

Helikites are probably the most commonly used worldwide at present, with thousands produced each year. Helikites are generally very small compared to other aerostats and fly in all but the most extreme weathers. They are a combination of a oblate-spheroid balloon and a kite. Thus both the wind and helium push Helikites upwards so they can fly to 6000ft with ease even with no ballonet. Helikites are very stable in foul and have introduced the concept of the "personal aerostat".

Spherical balloons have the lowest surface-area-to-volume ratio and they lift well in low or no wind. However, unless they are very large, in most winds they quickly begin to be pushed to the ground. In light winds, very large rounded balloons are used to lift people for recreational flight.

Blimp-shaped balloons were originally designed as barrage balloons just before the First World War. Thousands of blimps were used in both world wars, but they have changed little in design since the First World War. The British L.Z. type of the Second World War was based upon the French Caquot type of 1915. A British L.Z. barrage was sent to the USA in 1942 where it was copied and became the ZK Type made by the Goodyear Tire & Rubber Company. Today most blimps are used for advertising in fair weather. Some massive blimps are used for lifting radar or surveillance cameras.

Blimps are more or less sausage-shaped, to reduce frontal area and wind resistance. They have stern fins to keep the balloon pointing into the wind. When they are correctly made, blimps are more stable than spherical balloons; however, their large ratio of surface area to volume requires blimps to be large so that they can lift a sufficient payload to be efficient. Generally, blimps must be large also to cope with high winds. Their long, thin shape necessitates a device to equalize pressure in the envelope, called a ballonet, if they are to rise over 900 feet in altitude and to cope with large atmospheric temperature changes.

When set at an angle to the wind, blimps can produce aerodynamic lift especially from their stern fins. When blimps do this it is called "kiting". As the wind increases further this lift causes the stern to rise and the nose to lower. The low nose is further pushed down by the wind leading to an instability called "porpoising". To reduce porpoising the tethers are set to further raise the nose in high winds, however this increases the drag on the blimp causing the blimp to lose height and the tether to lay over to give "quatenary" problems. The handling and cost implications of the blimps large size means they are not commonly used by the general public. However, the military sometimes use large blimps for surveillance and radio relay due to their ability to stay in the air for long periods of time in reasonable weather.

Applications

Aerostats are used for lifting military airborne radar equipment, parachute training, for advertising, lifting meteorological equipment, raising antennas, gaining line of sight for ad hoc radio relay stations, lifting video equipment and digital cameras, for jungle marker balloon use and birdscaring.

A new, patented shape of aerostat is shaped like an airfoil. As opposed to most tethered aerostat shapes, the airfoil or "wing-shaped" aerostats can create their own lift and can stay aloft in winds up to 70+ knots. They are still filled with a lighter-than-air gas and tethered to a ground station. In this case, the ground station is relatively small and can be pulled behind a pick-up truck, hummer or similar. The payload is no longer attached to the bottom of the aerostat. It is floating below the aerostat providing improved performance.[3]

Gallery

See also

References

  1. ^ The Chambers Dictionary. Edinburgh: Chambers Harrap Publishers Ltd. 2000 [1998]. p. 541. ISBN 0-550-14005-X. "the gas-bag of a balloon or airship"  
  2. ^ The Oxford Illustrated Dictionary. Great Britain: Oxford University Press. 1976 [1975]. p. 281. "fabric enclosing gas-bags of airship"  
  3. ^ http://www.overheadcomm.com

External links

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